1. Field of the Invention
The present invention relates generally to varactor tuned LC circuits, and more particularly relates to voltage controlled oscillator (VCO) circuitry with dual varactor diodes having a common anode configuration.
2. Prior Art
Shown in FIG. 1 is a prior art high frequency tuning subcircuit wherein a positive control voltage Vc is applied to node 50 to vary the voltage at node 17. Optional capacitor 27 may be used to filter certain high frequency effects on node 50 to ground node 19, thereby maintaining an average direct current (DC) voltage at node 17 of approximately Vc. Varactor diodes 13 and 15 are connected in a common cathode manner at node 17 and are each bias controlled by Vc. Tuning inductor 82 in conjunction with the equivalent capacitance of the varactor diodes is used to set the resonant frequency of the subcircuit shown in FIG. 1. The subcircuit is subsequently coupled to an active circuit or drive circuit (not shown) using a coupling capacitor 37 at node 77. The frequency response of the subcircuit thereafter varies in response to the varying capacitances of varactor diodes 13 and 15, and in response to the charge transfer which occurs between tuning inductor 82 and varactor diodes 13 and 15.
It is an object of the present invention to provide varactor tuned LC circuits capable of providing a control signal or differential control signals for providing signal control of an oscillator circuit.
It is also an object of the present invention to provide a tuning circuit used with a voltage controlled oscillator wherein the control voltage is of a similar polarity as other circuits connected to the power source.
It is another object of the present invention to provide a tuning circuit for providing variable tuning of an associated circuit or active circuit such as voltage controlled oscillators, frequency filters, or the like.
It is yet another object of the present invention to provide a tuning circuit or subcircuit which provides variable capacitive tuning where the effective capacitance seen at the input node for the control voltage or modulation voltage into the tuning circuit is approximately twice the actual capacitance of the variable tuning device or approximately twice the actual capacitance of a varactor diode, particularly when matched devices are used in the tuning circuit or subcircuit.
In accordance with the teachings herein, the present invention provides a voltage controlled oscillator circuit in which the frequency determining circuit or tuning circuit comprises a pair of varactor diodes connected in a common anode configuration and optionally is fabricated in a single semiconductor substrate or package. The tuning circuit is implemented in conjunction with an inductor and coupling capacitors to form a variable frequency oscillator controlled by a control voltage. This circuit may also be used as a subcircuit which has applicability as the basic building block for a frequency synthesizer, a frequency filter, or the like. The circuit can be tuned such that frequency synthesizer, filter, or the like, operates at baseband frequencies, intermediate frequencies (I/F) or radio frequencies (R/F). These various frequency synthesizers, filters, and the like are presently used in cellular or hand held telephones, including digital and analog type telephones, and may be implemented in other radio or communication equipment in general.
The present invention offers, either separately or in combination, the following advantages:
1) a common anode connection node whereby the externally applied control voltage Vc applied to the control node will typically be a positive voltage with respect to a common ground node;
2) a circuit configuration which has basically symmetrical operating characteristics, particularly where matched component properties or matched components are used for corresponding circuit elements or components;
3) a tuning circuit or subcircuit capable of being isolated from the direct current (DC) ground of the active circuit or oscillator drive circuit thereby permitting separate DC bias of the tuning circuit or subcircuit and the active circuit or oscillator drive circuit while maintaining the symmetry of the circuit configuration; and,
when matched components are used in the tuning circuit or subcircuit, the following advantages are also of note:
4) the capacitance of the varactor diodes are effectively in parallel for the control or modulation voltages; and,
5) the varactor diodes are effectively in series for alternating current (AC) signals such as radio frequency R/F signals and for resonant frequency determination.
The foregoing objects and advantages of the present invention for a varactor tuned LC circuit may be more readily understood by one skilled in the art with reference to the following detailed description of the preferred embodiment thereof, as shown by the accompanying drawings wherein like elements are designated by identical reference numerals throughout the several views.